The present invention relates to a process for the separation of lead from a sulfidic concentrate and to a flash-smelting furnace for carrying out the process.
Most of the world's lead is produced from sulfidic concentrates by the sintering-shaft-furnace process. In the sintering machine, the concentrate is oxidized in order to remove the sulfur, and it is brought to a suitable particle size for shaft furnace reduction.
The greatest disadvantage of the process is its large amounts of waste gas, which are produced by both the sintering and the shaft furnace process. It has been estimated that process and ventilation gases, which contain sulfur dioxide and dust, are produced at a rate of about 670 kmol (15,000 Nm.sup.3) per one tonne of concentrate. The purification of the waste gases to meet the current requirements of environmental protection causes a significant increase in the costs of lead production.
The goal of recent research work has been to create a process in which sulfur dioxide is obtained in a concentrated form and the amount of dust-bearing waste gases is minimal. In principle, a single-stage process is possible for pure concentrates which contain very little quartz. A sulfidic lead concentrate is oxidized directly to metal in a single process stage. As a sub-reaction, the lead sulfide first oxidizes to oxide in accordance with the following reaction: EQU PbS+1/2O.sub.2 =PbO+SO.sub.2
Thereafter, the excess lead sulfide reduces the oxide to metallic form according to the following reaction: EQU 2PbO+PbS=3Pb+SO.sub.2
If the operating temperature of the process is below about 1373 K., sulfate and oxysulfates of lead are produced by the reactions instead of lead oxide. Metallic lead is produced when these compounds react with lead sulfide.
The single-stage lead production process is suitable for pure concentrates. Owing to the high mutual affinity of lead oxide and silica, the concentration of lead in the slag increases and the yield of metallic lead decreases when the concentration of quartz in the concentrate increases. Releasing the lead from the silicate requires so low an oxygen pressure that, in the presence of sulfur dioxide, lead sulfide is obtained instead of metallic lead.
At those temperatures and oxygen pressures which are used in direct production of lead, the zinc present in the concentrate oxidizes and passes into the slag. In order to maintain the melting point of the slag sufficiently low, the slag must be fluxed, which for its part increases the losses of lead into the slag.
Multiple-stage processes have therefore been applied to the treatment of the above-mentioned impure concentrates. It has been possible to eliminate the disadvantages of the sintering process, i.e. A dilute sulfur dioxide gas, the passing of lead oxide dust into the environment, the formation of sulfates, and difficulties in temperature control, by shifting to closed reactors, the product of which is a lead-oxide bearing melt. Such is, for example, the Kivcet process (FI Lay-Open Print No. 56028).
The vapor pressure of lead sulfide in particular, but also of lead oxide, is high at the operating temperatures of lead production processes. This is the reason for the large amounts of fly dust, which are typical of the process and very detrimental. In both a multiple-stage and a single-stage process there occurs volatilization of both sulfide and oxide of lead. The boiling point of lead sulfide is about 1610 K. and that of lead oxide about 1810 K., and so the gas may contain large amounts of the said compounds at the processing temperatures. The volatilized lead compounds leave the processing apparatus along with the sulfur-dioxide bearing gas.
Depending on the sulfur dioxide pressure, only the sulfide, sulfate and various oxysulfates of lead are stable below 1050-1150 K. For this reason, the dust separated from the cooled gas, the amount of the dust possibly representing a very high proportion of the lead amount fed into the process, primarily consists of these compounds. The amount of lead oxide is less. Feeding the fly dust to the oxide reduction stage is not possible owing to the sulfur present in it. During the reduction stage the sulfur would be reduced and would leave along with the gas in the form of lead sulfide. Likewise, the concentration of sulfur in the lead produced would be high. The most common method of treating the dust is to feed it, together with fresh concentrate, back to the oxidation stage. However, there is a disadvantage in the amount of energy required by the endothermal decomposition reactions of the sulfates and in the increased amount of gas in the process owing to the high rate of recycling of dust.
One of the main objectives in the development of the lead process has been to decrease the amounts of dust. One method for achieving this has been to cool the gas in the outlet section of the oxidation reactor in such a manner that the compounds of lead condense and fall back into the hot melt. This procedure is used in the Kivcet process when lead concentrate is oxidized. The returned of the cooled dust, which possibly contains sulfates, causes, however, additional consumption of heat, since the amount of dust is high, 25-40%.
Another method used in several processes in order to decrease the dust amount is to inject sulfide concentrate either into the surface or below the surface of the melt in the furnace. Thereby the sulfide is caused either to dissolve rapidly in the molten lead or to react with the lead oxide present in the slag, whereby the activity of the lead sulfide decreases and volatilization decreases.
Finnish Pat. No. 54147 discloses a process for the suspension smelting of sulfidic complex and/or mixed ores or concentrates by feeding a finely divided raw material, air or oxygen-enriched air, and possibly fuel into the upper section of the reaction zone in order to form a suspension, whereby the raw material in suspension is exposed to an oxidizing treatment at a high temperature in the upper section of the reaction zone and to a reducing or sulfidizing treatment in the lower section of the reaction zone in order to cause the non-volatile impurity minerals or impurity metals to pass back into the gas phase before the solid in the suspension separates and impinges on the melt surface below the reaction zone.
In this patent it is pointed out that often it is not possible to remove the lead quantitatively from the products of the smelting without effective reduction-sulfidization, since the lead oxide produced during the oxidation readily reacts in the shaft with the concentrate or the silicic acid of the additives fed. It is noted that, owing to the disadvantageous activity conditions, it is difficult to reduce the lead and to sulfidize it out from molten silicates.
None of the above-described processes is capable of decisively decreasing the dust problem in the lead production process. A large part of the lead content of the concentrate continues to leave along with the gas and is sulfated or sulfidized during the cooling of the gas.
The objective of the present invention is to eliminate substantially the dust problems occurring in the above-mentioned prior known processes and to provide a process for the separation of lead from a sulfidic concentrate.
A further objective of the invention is to provide a flash-smelting furnace intended for use in this new process, a furnace in which retention of the melt is not necessary and in which the dust amount carried along by the discharging gas in decisively less than in prior known corresponding apparatus.